Procedure and device to heat a reducing agent generation system

ABSTRACT

The invention concerns a procedure to heat a reducing agent generation system of an exhaust gas aftertreatment system of an internal combustion engine by combustion of fuel, whereby the reducing agent is produced for the selective catalytic reduction of nitrogen oxides in the exhaust gas of the internal combustion engine in the intermittently operated reducing agent generation system, whereby the reducing agent generation system consists of a plasma burner, a mixing chamber located downstream or upstream from the plasma chamber, an oxidation reformation unit as well as a nitrogen oxide storage/ammonia production unit and whereby nitrogen oxide is produced in a plasma in the plasma burner. The invention concerns additionally a corresponding device. 
     The task of the invention concerning the procedure is thereby solved, in that the combustion of the fuel is ignited by the plasma. For this reason, the heating of the catalytic components required at the start of the reducing agent generation system results through the combustion of fuel by means of a burner functionality, which consists of a fuel metering mechanism and the existing plasma burner. A special heating burner as an auxiliary mechanism can be omitted.

BACKGROUND

In the German patent DE 199 22 961 C2 an emission control system for thepurification of the exhaust gas of a combustion source, especially theinternal combustion engine of a motor vehicle, is described at least bythe nitrogen oxides contained therein with an ammonia producingcatalytic converter for the production of ammonia using components of atleast one part of the exhaust gas emitted from the combustion sourceduring the ammonia producing operational phases and with a nitrogenoxide reduction catalytic converter subsequently connected to theammonia production catalytic converter for the reduction of the nitrogenoxides contained in the exhaust gas emitted from the combustion sourceusing the ammonia produced as a reducing agent. Provision is madethereby for a nitrogen oxide production unit external to the combustionsource for the enrichment of the exhaust gas supplied to the ammoniaproduction catalytic converter with the nitrogen oxide it producesduring the ammonia producing operational phases. A plasma generator isproposed, for example, as a nitrogen oxide production unit for theplasma engineered oxidation of the nitrogen contained in a gas stream,which is supplied, to nitrogen oxide. The hydrogen required for theammonia production is produced during the ammonia production operationalphases by the operation of the combustion source with a rich, i.e. fuelrich air ratio.

A plasma chemical procedure to produce a hydrogen rich gas mixture isdescribed in the patent WO 01/14702 A1. With the procedure, a richfuel-air-mixture is dealt with in an arc, preferably under POxconditions.

In order to avoid the transport of an additional resource, a plasmaprocedure was proposed by the applicant in a still unpublished writingfor the on-board-generation of reducing agents. In so doing, necessaryammonia from non-toxic substances are produced according to need in thevehicle and subsequently supplied to the SCR-process. An acceptablesolution with regard to the fuel consumption is afforded thereby by anintermittently operated procedure for ammonia production, which likewiseis proposed in this writing. This procedure is denoted as follows as theRGS-procedure (Reductant Generating System) or the reducing agentgenerating system.

A disadvantage of this procedure is that especially in the startingphase the reducing agent generation system (RGS) only very slowlyachieves an adequately high operating temperature, at which an optimalfunctionality is guaranteed. The strategy up to the present makesprovision for a burner functionality, which makes possible for thesystem to be made operational, especially the catalytic components forpartial oxidation at approximately 500E C and the ammonia productionunit at approximately 250E C. For that purpose, provision is made for adiesel fuel combustion in a flame to be contingently supported by acatalytic combustion within the catalytic components.

A disadvantage is that an additional mechanism is required to heat thereducing agent generation system (RGS) up to full operating status.

It is therefore the task of the invention to provide a procedure, inwhich on the one hand allows for the quick achievement of an optimaloperating temperature of the RGS-unit and on the other hand minimizesthe expenditure for auxiliary mechanisms. It is furthermore the task ofthe invention, to provide at this point a suitable device.

SUMMARY

The invention concerns a procedure to heat a reducing agent generationsystem of an exhaust gas aftertreatment system of an internal combustionengine by combustion of fuel, whereby the reducing agent for theselective catalytic reduction of nitrogen oxides in the exhaust gas ofthe internal combustion engine is produced in the intermittentlyoperated reducing agent generation system, whereby the reducing agentgeneration system consists of a plasma burner, a mixing chamber locatedeither upstream or downstream from the plasma chamber, an oxidationreformation unit as well as a combined nitrogen oxide storage/ammoniageneration unit and whereby nitrogen oxide is produced in a plasma inthe plasma burner.

The invention additionally concerns a device to heat up a reducing agentgeneration system of an exhaust gas aftertreatment system of an internalcombustion engine by combustion of fuel, which can be metered in by wayof a fuel metering mechanism, whereby the reducing agent generationsystem consists of a plasma burner, a mixing chamber located downstreamor upstream from the plasma burner, an oxidation reformation unit aswell as a combined nitrogen oxide storage/ammonia generation unit forthe intermittent production of ammonia from fuel and air and/or exhaustgas for the selective reduction of nitrogen oxides in the exhaust gas ofthe internal combustion engine.

In context with future legal regulations with regard to nitrogen oxideemissions from motor vehicles, an exhaust gas aftertreatment isrequired. The selective catalytic reduction (SCR) can be deployed toreduce the nitrogen oxide emissions (denitrogenation) of internalcombustion engines, especially of diesel motors, with chronologicallypredominantly lean, i.e. oxygen rich exhaust gas. In so doing, a definedamount of a selectively acting reducing agent is added to the exhaustgas. This can, for example, be in the form of ammonia, which is meteredin directly as a gas, which is derived from a precursor substance in theform of urea or from a urea-water-solution (HWL).

The task of the invention concerning the procedure is thereby solved, inthat the combustion of the fuel is ignited by the plasma. In so doing,the required heating of the catalytic components at the start of thereducing agent generation system can result by combustion of fuel bymeans of a burner functionality, which consists of a fuel meteringmechanism and the plasma burner, which is present in any case. Anindependent heating burner as an auxiliary mechanism can be omitted.

If the fuel is supplied to a mixing chamber upstream from the plasmachamber, a reliable ignition of the fuel for the burner operation can beachieved by utilization of the entire length of the plasma. Furthermore,the fuel-air-mixture in the hydrogen production phase runs through theentire area previously heated up by the plasma, whereby the fuel isdefinitely vaporized.

Provision is made in one form of embodiment for the fuel to be suppliedto one of the mixing chambers downstream from the plasma chamber and forthe plasma to be fed into the mixing chamber lying downstream. In sodoing, the mixing chamber downstream is heated, in order to reliablyvaporize the fuel in the hydrogen production phase, and on the otherhand the fuel can be ignited to heat up the catalytic components. In thephase of the nitrogen oxide production the long plasma area causes ahigh nitrogen yield.

If nitrogen oxide in the plasma of the plasma burner is produced in anitrogen oxide production phase, and in a subsequent ignition phase fuelis metered into the mixing chamber (41, 42) located upstream ordownstream from the plasma chamber and is ignited by the plasma (30),and in a subsequent heating phase the plasma is turned off, whereby fuelis additionally metered into a mixing chamber located either upstream ordownstream from the plasma chamber and combusted, and in a subsequenthydrogen production phase the fuel combustion is completed and fuel isvaporized in the mixing chamber located upstream or downstream from theplasma chamber, the burner function required to heat up the reducingagent generation system as well as the production of nitrogen oxide andhydrogen for the production of the reducing agent can be implementedwith a simple and cost effective system.

A reliable ignition of the fuel-air-mixture for the burner operation isachieved, in that the plasma is additionally operated for a selectivetime duration, preferably for a time duration between one and fiveseconds, after the beginning of the metering in of the fuel.

If the fuel is vaporized before combustion on a hot surface area, whichwas heated up during the nitrogen oxide generation phase of the mixingchamber located either upstream or downstream from the plasma chamber,an atomization of the fuel can occur with a fuel pressure of maximally 4bar as it exists in normally deployed systems. Without the auxiliaryheating, a fuel pressure of typically 10 bar would be required, whichwould mean additional complexity of components.

An improved vaporization of the fuel can be achieved, in that the fuelsupplied and/or the air supplied is heated in the counter flow on theoutside cladding of the plasma burner.

The task concerning the device of the invention is solved, in thatprovision is made for a plasma of the plasma burner to ignite the fuel.In this way the deployment of a separate ignition mechanism isunnecessary. Such a mechanism would include a glow plug or a separateburner unit to heat the catalytic components of the reducing agentgeneration system.

If the plasma of the plasma burner is designed for one or more plasmajet zones, the fuel-air-mixture can be reliably ignited for the burneroperation, especially with the execution with several plasma jet zones.

If a housing of the plasma burner has a tapered interior area in thearea subsequent to the plasma chamber in the direction of the gas flow;in collaboration with the gas flow, the plasma can extend itself acrossan especially long area and can develop a plasma jet escaping from thehousing.

If the plasma jet zones are disposed tangentially around an ignitionarea or linearly along an ignition area, the fuel-air-mixture can beimpressed with a swirl, which leads to a particularly good mixture or along ignition area is developed, which likewise improves the combustion.

If provision is made for a flame holder between the fuel meteringmechanism and the ignition area, the flame burns more stably and anextinguishment can be avoided.

If provision is made for two fuel metering mechanisms for the fueldelivery during the heating up and for the fuel delivery during a H₂/COproduction phase, provision can be made for an advantageous arrangementin each case for both modes of operation. If provision is made for ajoint fuel metering mechanism, the arrangement is especially costeffective.

BRIEF DESCRIPTION OF THE DRAWTNGS

The invention is explained in more detail in the following manner usingthe examples of embodiment depicted in the figures. The followingfigures are shown:

FIG. 1 a schematic depiction of an exhaust gas aftertreatment system ofan internal combustion engine with a reducing agent generation system,

FIG. 2 a plasma burner for the reducing agent generation system

FIG. 3 the plasma burner in an alternative embodiment.

DETAILED DESCRIPTION

FIG. 1 shows schematically the technical layout using the example of adiesel motor, in which the procedure according to the invention can beapplied. An internal combustion engine 50 is depicted with an exhaustgas duct, a SCR-catalytic converter connected to the duct for theselective catalytic reduction (SCR) of the exhaust gas of the internalcombustion engine 50, an exhaust gas outlet connected to theSCR-catalytic converter and a reducing agent generation system 1. Thereducing agent generation system 1 provides for the production ofammonia, which is stored in the SCR-catalytic converter and convertsnitrogen oxides from the exhaust gas of the internal combustion engine50 to water and nitrogen. The reducing agent generation system 1consists of a plasma burner 10, a subsequently connected oxidationreformation unit 60 and a nitrogen oxide storage/ammonia production unit61, from which the ammonia can be metered in by way of a reducing agentfeed 62 into the exhaust gas duct 53 in front of the SCR-catalyticconverter. Air and/or exhaust gas can be fed to the plasma burner by wayof a first air feed 13; and by way of a fuel metering device 40, fuel asa basic material for the production of ammonia can at least periodicallybe fed.

The production of ammonia results within the reducing agent generationsystem 1, in which nitrate monoxide NO in a lean phase (λ>1) is producedin a plasma 30, which is not depicted here, within the plasma burner 10from air and/or exhaust gas. The nitrogen oxides flow through theadjoining oxidation reformation unit 60 and are subsequently deliveredin the example shown to a combined nitrogen oxide storage/ammoniageneration unit 61 and stored there. In a second operating phasesubsequently connecting to the lean phase, the rich phase (0.33<λ<1),liquid fuel is metered into the plasma burner, vaporized and convertedin the oxidation reformation unit 60 to a gas mixture containinghydrogen and carbon monoxide, which subsequently converts the previouslystored nitrogen oxides to ammonia in the nitrogen oxide storage/ammoniaproduction unit 61. This gaseous ammonia, which has been produced, isthen metered into the exhaust gas stream of the exhaust gas duct 53 infront of the SCR-catalytic converter.

As the SCR-catalytic converter 54 possesses an ammonia storagecapability, it is also possible by means of an intermittent procedurefor ammonia production to achieve continuously the reduction of thenitrogen oxides by means of the SCR-process in the exhaust gas stream.In so doing, catalytic converters from, for example, titanium dioxide(TiO₂) and vanadium-pentoxide (V₂O₅) convert the nitrogen oxides withthe ammonia generated at a high rate in the temperature range between150° C. and 450° C.

In order to achieve in the starting phase a quick heating of thecomponents of the reducing agent generation system 1, especially by theoxidation reformation unit 60 and the nitrogen oxide storage/ammoniaproduction unit 61, the plasma burner 10 has according to the inventiona functionality for the combustion of fuel, which is described in thefollowing way. A quick operating status of the reducing agent generationsystem 1 is achieved with this burner functionality, so that a highammonia generation rate is accomplished already very early.

FIG. 2 shows a plasma burner 10 with a housing 20, in which an electrode14 is attached to an electrode holder 12, which is electricallyseparated from the housing 20 by an insulator 15. Between the electrode14 and the housing 20 the plasma 30 can be ignited in a plasma chamber31 and kept going, in that a high voltage is impressed by a high voltagesource 17 between a first high voltage terminal 11 at the electrodeholder 12 and a second high voltage terminal 18 at the housing. The highvoltage can thereby be a DC voltage or a high frequency voltage. Air byway of the first air feed 13 or additional ones, for example executed bya second air feed 16, is delivered to the plasma chamber 31. The housing20 has in the direction of flow in an area downstream from the plasmachamber a tapered interior area, so that the plasma develops a plasmajet, which extends beyond the housing 20 and develops an escaping plasmajet 32. Depending upon the form of the tapered interior area 21 and thegas flow, the plasma 30 can in the area of the tapered interior area 21can be dead or partially active, whereby a current-carrying plasma has ahigher temperature. In the area of the escaping plasma jet 32, the fuelmetering mechanism 40 is disposed, which can meter fuel into asubsequently connected mixing chamber.

In a first mode of operation of the plasma burner 10, air by means of afirst air feed 13 and a second air feed 16 is delivered past theelectrode 14 to the plasma chamber 31. By means of the burning plasma inthe succeeding area, nitrogen oxides are produced, which in theunspecified additional components of the reducing agent generationsystem 1 are used for the production of ammonia.

In a second mode of operation of the plasma burner 10, the plasma isproduced and by way of the fuel metering mechanism 40 meters fuel intothe subsequently connected mixing chamber 41. The fuel vaporizes on thehot cladding of the subsequently connected mixing chamber 41, is ignitedby the escaping plasma jet developed in the plasma 30 and forms a burnerflame. If the fuel is reliably ignited, the high voltage source 17 canbe turned off and the additionally metered in fuel ignites at theexisting burner flame. Provision can be made for a flame holder tostabilize the operation. This mode of operation serves to heat up thecatalytic components of the reducing agent generation system 1. Thesecond mode of operation is ended, in that the fuel supply isinterrupted, so that the burner flame extinguishes.

In a third mode of operation, when the plasma 30 is turned off, fuel ismetered into the hot subsequently connected mixing chamber 41. The fuelvaporizes and can be converted to hydrogen, which serves to produceammonia, in the oxidation reformation unit 60, which is not depictedhere. Additionally by-products arise like carbon monoxide andhydrocarbons.

In a modified embodiment of the plasma burner 10 depicted in FIG. 3, thefuel metering mechanism 40 is disposed in the direction of gas flow infront of the plasma chamber 31. In the second mode of operation of theplasma burner 10, fuel is metered into a mixing chamber 42 locatedupstream from it with the fuel metering mechanism 40. The fuel vaporizesthere on walls heated in the previous first operational phase andignites in the plasma 30 located downstream in the direction of gasflow. For an improvement in the vaporization, the incoming air and/orthe fuel can thereby be warmed at the housing 20 in accordance with thecounter current principle. The advantage of this arrangement is that thefuel runs through a long plasma zone and reliably ignites. If the fuelignites reliably, the high voltage source 17 can be switched off and thefuel, which is additionally metered in, ignites at the existing burnerflame. Provision can be made for a flame holder to stabilize theoperation. This mode of operation serves to heat up the catalyticcomponents of the reducing agent generation system 1. The second mode ofoperation is ended, in that the fuel supply is interrupted, so that theburner flame extinguishes.

In the third mode of operation, when the plasma 30 is turned off, fuelis metered into the hot mixing chamber 42 located upstream. The fuelvaporizes there and in the plasma chamber 31 heated by the plasma 30 aswell as in the tapered interior area 21 of the housing 20.

The previously described procedural variations of the secand mode ofoperation of the plasma burner 10 are implemented with the executions ofthe device previously described, especially during a cold start and/orwhen restarting the exhaust gas aftertreatment system. As a result aquick heating of the reducing agent generation system 1 with itscomponents can be achieved, whereby the supplying of ammonia as areducing agent is accelerated and a quick system start of the exhaustgas aftertreatment system is made possible.

Basically the device and the procedure can be deployed in all motorvehicles with diesel or other lean engines, which are operated withother fuels, in which a reducing agent generation system 1 is deployed.

The invention claimed is:
 1. A method of heating up a reducing agentgeneration system of an exhaust gas aftertreatment system of an internalcombustion engine by combustion of fuel, the reducing agent generationsystem comprising a plasma burner, a mixing chamber located downstreamor upstream from the a plasma chamber, an oxidation reformation unit,and a combined nitrogen oxide storage and ammonia production unit, themethod comprising: producing the reducing agent for selective catalyticreduction of nitrogen oxides in an exhaust gas of the internalcombustion engine in the intermittently operated reducing agentgeneration system; producing nitrogen oxide in a plasma in the plasmaburner, wherein the combustion of the fuel is ignited by the plasma;producing nitrogen oxide in a nitrogen oxide production phase, in theplasma of the plasma burner; metering fuel into the mixing chamber andin a subsequent ignition phase the fuel is ignited by the plasma, and ina subsequent heating phase the plasma is turned off; and additionallymetering fuel into the mixing chamber and combusting the fuel and in asubsequent hydrogen production phase, ending the fuel combustion andvaporizing the fuel in the mixing chamber.
 2. A method according toclaim 1, wherein the step of metering fuel into the mixing chamberincludes delivering fuel is delivered to the mixing chamber upstreamfrom the plasma chamber.
 3. A method according to claim 1, wherein thestep of metering fuel into the mixing chamber includes delivering fuelis delivered to the mixing chamber downstream from the plasma chamberand feeding the plasma is fed into the mixing chamber.
 4. A methodaccording to claim 1, wherein after beginning to meter in the fuel, theplasma is additionally operated for a selective time duration,preferably for a time duration between one and five seconds, after abeginning of the metering in of the fuel.
 5. A method according to claim1, wherein the fuel is vaporized before combustion on a hot surfacearea, which was heated during the nitrogen oxide production phase, ofthe mixing chamber located either downstream or upstream from the plasmachamber.
 6. A method of heating up a reducing agent generation system ofan exhaust gas aftertreatment system of an internal combustion engine bycombustion of fuel, the reducing agent generation system comprising aplasma burner, a mixing chamber located downstream or upstream from theplasma chamber, an oxidation reformation unit, and a combined nitrogenoxide storage and ammonia production unit, the method comprisingproducing the reducing agent for selective catalytic reduction ofnitrogen oxides in an exhaust gas of the internal combustion engine inthe intermittently operated reducing agent generation system; andproducing nitrogen oxide in a plasma in the plasma burner, wherein thecombustion of the fuel is ignited by the plasma, wherein the fuel or theair delivered in a counter flow is warmed on an outside cladding of theplasma burner.
 7. A device to heat a reducing agent generation system ofan exhaust gas aftertreatment system of an internal combustion engine bycombustion of fuel, which is metered in by way of a fuel meteringmechanism, whereby the reducing agent generation system comprises aplasma burner, an oxidation reformation unit, and a combined nitrogenoxide storage/ammonia production unit for the intermittent production ofammonia from fuel and air or exhaust gas for selective catalyticreduction of nitrogen oxides in an exhaust gas of an internal combustionengine, wherein a plasma of the plasma burner ignites the fuel, whereinthe plasma burner is configured: to produce nitrogen oxide in a nitrogenoxide production phase, in the plasma of the plasma burner; to meterfuel into the mixing chamber, and in a subsequent ignition phase, havethe fuel is ignited by the plasma, and in a subsequent heating phase,have the plasma is turned off; and additionally to meter fuel into themixing chamber and combusting combust the fuel and in a subsequenthydrogen production phase, to end the fuel combustion and vaporizing thefuel in the mixing chamber.
 8. A device according to claim 7, wherein ahousing of the plasma burner has a tapered interior area in the area inthe direction of gas flow succeeding the plasma chamber.
 9. A deviceaccording to claim 7, further including a flame holder between the fuelmetering mechanism and the ignition area.
 10. A device according toclaim 7, further including two fuel metering mechanisms or a joint fuelmetering mechanism for the fuel delivery during heating and for the fueldelivery during a H₂/CO production phase.
 11. A device according toclaim 7, wherein the plasma of the plasma burner is developed into oneor more plasma jet zones.
 12. A device according to claim 11, whereinthe plasma jet zones are disposed tangentially around an ignition areaor linearly along an ignition area.